Functionality to handle user terminal mobility is a fundamental component in cellular networks. From a service quality perspective, such functionality must ensure that service continuity is maintained as users move from one cell to another during an active session, and that each new session is established in a sufficiently good radio environment. From a spectral efficiency perspective, such functionality should ensure that an active user is always served by the most appropriate base station or base stations, which typically means the closest base station(s) in a radio sense. The process of modifying the number or identity of base stations serving a given user during an active session is commonly referred to as (serving) cell change or “handover”.
The provision of diversity against fading is a key method for improving the performance and efficiency of cellular networks. While there are numerous micro diversity techniques aimed at mitigating the effects of Rayleigh fading, (examples include receiver diversity, transmit diversity, interleaving and frequency hopping), macro diversity to combat the impact of shadow fading has fewer options. In order to obtain a macro diversity gain the possibility for simultaneous transmission and/or reception of related information in different base stations must be involved in some way. Macro diversity is particularly beneficial near the cell border, where it can dramatically improve service coverage, especially when the shadow fading from different base stations is uncorrelated.
A multi-carrier system utilizes multiple radio transmission resources to transmit data to and/or receive data from a given user terminal. Multi-carrier often means that multiple carrier frequencies are used, but more generally it can include all systems where a plurality of radio transmission resources can be simultaneously allocated to a given user terminal. Examples of radio transmission resources well-known in the art are frequencies, time slots, Orthogonal Frequency Division Multiplexing (OFDM) chunks, channelization codes, etc.
In existing cellular systems, including multi-carrier systems, the handover process is typically classed as “hard” or “soft”. The hard handover procedure is schematically illustrated in FIGS. 1A to 1C for a multi-carrier system. In FIG. 1A, a mobile user terminal 10 present in a cell 25 of a serving base station 20 has two radio transmission resources 22, 24 simultaneously allocated for conducting uplink or downlink communication with the base station 20. The mobile user terminal 10 then starts to move away from the serving base station 20 and into a cell 35 of another base station 30. FIG. 1B illustrates the situation where the mobile terminal 10 is present within the coverage areas of both cells 25, 35. At this position, the resources 22, 24 allocated by the old base station 20 are still utilized for communication. If the movement continues, as in FIG. 1C, the single serving base station 20 passes on the connections in their entirety to another serving base station 30. The mobile user terminal 10 will then have two radio transmission resources 32, 34 allocated for communication with the new base station 30.
Soft handover in a multi-carrier system is schematically illustrated in FIGS. 2A-2C. In FIG. 2A, a mobile user terminal 10 has been simultaneously allocated two radio transmission resources 22, 24 for conducting uplink or downlink communication with a base station 20. This mobile user terminal starts to move away from the cell 25 served by the current active base station and into a new cell 35 of a new base station 30, see FIG. 2B. At this instance, two radio transmission resources 32, 34 are also simultaneously allocated to the user terminal 10 for communication with the new base station 30. This means that a so-called active set of multiple base stations 20, 30 maintains simultaneous connections 22, 24, 32, 34 with the mobile user terminal 10. Duplicate information is always simultaneously transmitted and/or received in the multiple base stations 20, 30 using the respective allocated resources 22, 24 and 32, 34. This means that the same raw (bit) data is transmitted on the allocated resources 22, 24 and 32, 34 in the two different cells 25, 35, though different physical layer processing, e.g. scrambling, of the raw data can be used in the two cells 25, 35. As the mobile user terminal 10 continues its movement into the new cell 35, see FIG. 2C, the old base station 20 is removed from the active set and the allocation of its resources 22, 24 is withdrawn. The user terminal 10 will then only have two simultaneously allocated radio transmission resources 32, 34 for conducting data communication.
Another less common handover method that can be regarded as a hybrid between soft and hard handover is “fast switching”. It is similar to soft handover in that a potentially active set of multiple base stations is maintained, but only one base station in the potentially active set actually transmits or receives at any point in time, the choice of active base station being based on the instantaneous radio conditions.
Soft handover and fast switching solutions typically give better service coverage and a more robust handover process than hard handover because they inherently include a macro diversity gain since related, in this case duplicate, information is simultaneously transmitted and/or received in the multiple base stations. However, they involve higher complexity and cost in infrastructure and user terminals, and they demand more transmission and air interface resources (the number of resources allocated to the terminal is doubled during the handover). Their introduction is in many cases simply unfeasible.
Hard handover is easier and cheaper to implement and it utilizes fewer resources than soft handover and fast switching, but achieving the requisite robustness and service coverage can be difficult due to the lack of macro diversity.
An example of the service coverage difficulties experienced with hard handover can arise when a cell change is performed during a file download or similar data transfer. Data already in the buffer for the source cell may either need to be emptied before the handover can be performed, which delays the handover procedure, or discarded and then retransmitted in the target cell. Either way, service quality and system performance are adversely affected.